1. Field of the Invention
This invention relates generally to lithographic processing. More particularly, this invention relates to an improved split reaction mass system for counter-balancing stage movement during lithographic processing.
2. Background Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer. During lithography, a wafer is disposed on a wafer stage and held in place by a chuck. The chuck is typically a vacuum or electrostatic chuck capable of securely holding the wafer in place. The wafer is exposed to an image projected onto its surface by exposure optics located within a lithography apparatus. While exposure optics are used in the case of photolithography, a different type of exposure apparatus can be used depending on the particular application. For example, x-ray, ion, electron, or photon lithographies each may require a different exposure apparatus, as is known to those skilled in the relevant art. The particular example of photolithography is discussed here for illustrative purposes only.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to remove exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface, or in various layers, of the wafer.
Step-and-scan technology works in conjunction with a projection optics system that has a narrow imaging slot. Rather than expose the entire wafer at one time, individual fields are scanned onto the wafer one at a time. This is done by moving the wafer and reticle simultaneously such that the imaging slot is moved across the field during the scan. The wafer stage must then be stepped between field exposures to allow multiple copies of a reticle pattern to be exposed over the wafer surface. In this manner, the sharpness of the image projected onto the wafer is maximized.
While using a step-and-scan technique generally assists in improving overall image sharpness, image distortions may occur in such systems due to movement of the entire system caused by the acceleration of the reticle stage or wafer stage. One way to correct this is by providing a counter balance (also referred to as a reaction mass) to minimize the movement of the lithographic system upon acceleration of a stage. Typically, counter balance mechanisms are guided by bearings or flexures.
When bearings are used, a number of bearings are needed to guide the reaction mass (e.g., some are needed underneath the reaction mass, some are needed on the sides, etc.). With a split reaction mass stage, where at least two reaction masses are used, many more bearings are needed. Although various types of bearings can be used (e.g., ball bearings, roller bearings, wheels, etc.), gas (or air) bearings are preferred in lithography systems because of good rectilinear motion. The extremely low friction of gas bearings also conserves momentum, minimizing motor size. In addition, transmitted vibration is significantly reduced when using gas bearings because air is used instead of a solid object such as a ball. Potential contaminants, such as the lubricant in a ball or roller bearing are not present with gas bearings. However, gas bearings are not compatible with high vacuum lithography systems for various reasons. Dynamically sealing against gas leakage into the vacuum chamber requires at least two pre-vacuum grooves in each cylindrical air bearing, which in turn demand additional vacuum pumps, resulting in an expensive system. The dynamic nature of the seal can result in some leakage of air bearing gas into the vacuum chamber, which increases the required size of the main vacuum pumps. Potential failure of the seal poses a high risk of catastrophic contamination within a controlled environment.
Flexures in the shape of thin plates may be used to guide the motion of a reaction mass. Typically, one end of a flexure is coupled to a protrusion of a reaction mass and the other end is coupled to another entity, such as a baseframe. In this way, both ends of a flexure are constrained so that the flexure cannot rotate upon movement of the reaction mass. Flexure plates usually include one or more groove-like channels at each end for flexibility in supporting the reaction mass. The channels can be angular, rounded, or of any shape that will allow flexibility in the flexure. Flexures, as opposed to gas bearings, are useful in guiding reaction masses in high vacuum environments because flexures are less expensive and do not pose a contamination risk. Flexures, however, present a variety of problems of their own, as is discussed below.
What is needed is a counter balance system used in conjunction with linear stages that stabilizes a lithographic system during processing, without the deficiencies associated with counter balance systems described above.